Production of calcium metal



3,208,845 PRODUCTION OF CALCIUM METAL Franz Kaess, Traunstein, and Heinrich 0. A. Rock, Trostberg, Germany, assignors to Siiddeutsche Kalkstickstolf- Werke Aktiengesellschaft, Trostberg, Germany No Drawing. Filed June 7, 1962, Ser. No. 200,665

7 Claims. (Cl. 75-67) The invention relates to the production of calcium metal by thermal dissociation of calcium carbide.

Such thermal dissociation at 1500 to 2000" C. in vacuo has already been disclosed in US. Patent No. 984,503. The react-ion is represented by the equation If technical calcium carbide is used, several side reactions take place which affect the purity of the produced calcium metal. The reason is that commercial calcium which reacts inthe opposite direction to form again carbide and calcium oxide at lower temperature or when the calcium is condensed. Such CaC, and CaO enter,

therefore, as impurities into calcium obtained by reaction (I). In the temperature range of 1200 to 1 800" C., the equilibrium pressures of both reactions are not much different so that both reactions take place side by side in the process as carried out on a technical scale. Attempts have been made to recover separately the products of the two reactions; such fractionated condensations have been disclosed in the German Patent No. 1,016,942, and in US. Patent No. 2,839,380. It is a principal object of the present invention to provide a method for producing I pure calcium from commercial calcium carbide without the complicated and not quite satisfactory fractionated condensation.

Other objects and advantages will be apparent from a consideration of the specification and claims.

In accordance with the present invention, the gaseous reaction products obtained when calcium carbide is heated in vacuo, are passed through a bed of granular carbonlhaving a loose, porous structure, a grain size of about 0.5 to 30 mm., and a large surface, preferably at least 5000 cm. /g. Reaction mass and carbon bed are maintained above the temperature at which reaction (2) has a higher equilibrium pressure than reaction (1). As shown by thermodynamic calculations, see, for instance,

the paper by C. Ma-ron, Zur. Priklad. Chim., vol. 30' 1957), p. 851, said condition is satisfied at a temperature above 1100" C.

We prefer to use temperatures in excess of 1500 C. not only in order to ensure a sufliciently great equilibrium pressure differential but also to obtain a technically suitable reaction rate. A third esential condition consists in maintaining in the first reaction step an absolute pressure which is lower than the equilibrium pressure of reaction (2) but higher than the equilibrium pressure of reaction l The total effect of the three features to be maintained in the first step of the thermal dissociation of technical calcium carbide is: Reaction (1) is suppressed and reaction (2) proceeds, whereby the gaseous mixture of Ca and CO passes through the carbon bed in which, due to the chosen reaction conditions, Ca is absorbed, probably with formation of pure CaC while the CO escapes. As a result of this procedure, the first phase of the reaction consists in reacting the CaO, which is contained as impurity in technical CaC with said CaC and in ensuring that only the CO escapes, and

Patented Sept. 28, 1965 that the thermal dissociation proper of the CaC takes place in the second phase only after all the C210 present has been reacted. Said second reaction phase is started very simply by lowering the absolute pressure, at constant temperature, below the value of the equilibrium .pressure of reaction (1); by all means, the pressure in said first reaction phase shouldbe always below 50 mm.

Hg. The same result is obtained when at constantpressure the temperature is increased until the equilibrium pressure of reaction (I) is higher than the absolute pressure. The described process allowsof producing purecalcium metal which is free ofcontamination with CaC,

and CaO.

The carbon layer which has to be passed by the reaction gas, may be provided in .various ways. A suitable bed is the residue of reaction (1) because it is a loose spongy material. -It may be used in the same grain size as the carbide charge, preferably about 3 to 30 mm. Coke and/or carbon of the same grain size may be used but in the case of coke the calcium may take up additional impurities from the volatile constitutents of the coke ash. The height of the carbon layer must be at least 50 mm. but should not exceed A of the height of the carbide layer. An increased effect of the carbon is accomplished by mixing granular carbon into the calcium carbide charge so that several small carbon particles adjoin each carbide grain. In this case, the grain sizes and amounts must be so adjusted that the carbon particles have about /2 to Vs the size of the carbide particles and fit into the interstices between the carbide particles. In this manher, the carbon layer may be mixed with the carbide charge, though a pure carbon layer may be additionally employed.

'In another embodiment of the invention, powdery carbide is mixed with powdery carbon and the mixture is compressed to pellets or briquets, which are then heated in a first and second reaction step, whereby also a surface layer of pure carbon may be additionally provided.

As reactor, we may use, for example, graphite crucibles equipped with suitable outlets for the removal of.

Temperature 0.- l, 400 1, 600 Pressure:

Lower limit mm. Hg" 0. l2 0. 9 4. 6 Upper limit mm. Hg 0. 46 7. 0 65 in the above table in order to decompose the calcium carbide in the second stage of the reaction.

The known procedures for the preparation of calcium from calcium carbide operate at such low pressures where the reactions (1)and (2) proceed simultaneously; There was no teaching as to how to utilize the calcium produced by reaction (2) or to separate it from carbon monoxide.

Particularly, the temperature-pressure relationship had not been recognized which makes it possible to carry out reaction (2) while suppressing reaction (1). The drawbacks of the prior art methods are avoided according to the invention by a two-step process which-presents the advantages of a simpler ered with a 200mm. high layer of residual graphite, grain size 310 10 'mm., through which the gases developed in thecarbide charge had to pass. The weight proportionas calcium carbitleto graphite was 20:3.

.The charge including the graphitelayer was heated at 1600 C. whilelthe reactor'was evacuated to a pressure of mm. Hg; Said temperature was maintained for 3 hours whereby towards the end of this first .reaction stage the pumping'power had to-be reduced to avoid lowering the'fpressure below theadmissible range;

- .Subsequently, thesecondreaction stage was started at the same temperature of 1600' C. by applying full power of the vacuum pump system, After V4 hour, the

pressure had dropped to 0.01 mm. Hg,awhencupon the a calcium carbide wasdecomposedat 1600' C.,' during a periodofGhours;

. From- 20; parts by weight of carbide, there were obtained :8 parts ofcrude metallic 95% calcium which contained still 1.5% CaC,, 3% C110, and traces of Al, "Si, and Fe from the impurities of the original carbide.

On distillation; cmde calcium produced 10.3 parts of pure calcium containing only 0.08% 'CaCj, correspond-- ing to a yield of 51- percent of pure calcium, calculated on calcium'carbiden We claim: V t

v 1 A process v:for producing calcium by thermal dissociation of commercial calcium carbide which comprises heating a charge consisting of a lower bed of. calcium carbide, ,which contains calcium oxide, and an upper layer of carbon particles having a large surface anda Y grain size of about'0.5 to 30'mm., in a first steplat a temperatureofvabout 1500 to 1800" .C. and at a pressure which is above pressure .of the reaction 1 I. cacgcnzc but below the equilibrium pressurelof the reaction f composition of Calcium Carbide.

so as toensure substantial suppression'of reaction(z-l)- in said first step, the-height of said carbon layerbeing substan-tiallysufiicicnt toabso'rb the calcium developed in r reaction (2) in said first step,. subsequently, after reaction-(2) has been substantially terminatedycontinuing heating said" charge at a temperatureiwithin said temperature range at a. pressure ';bel'ow. t' h e equilibriumpressure of reaction (1), thereby dissociating the-calcium carbide and. evaporating the dissociated v calcium: as: well as the calcium absorbed inzsaid 'uppercarbon layer, and

condensing said evaporated calcium. v r,

2. A process'as claimediin claim l wherein" theftemperature in said second step is -substantially the same; as in said first.step and-the pressure isr'educed'below the v dissociation pressure of the calcium carbideat said ternperature. i

3. A process as claimed in -;cla im 1 wherein said second reaction step is carried out at a higher'temperat'ure, within said temperature range, tha-n'the lirstreaction step at substantially the pressure of, said first step.

4. A process as claimedin claim 1 wherein the presv sure in the first reaction stepis below mm. Hg and is lower in the secondrcaction'step thaninthe first reaction step.

5. A process as claimed inclaim 1 wherein saidupper layer has a thickness. ofat least 501mm; and; not morev than 5 the height of the carbide charge. I

" 6. A process as claimed in claim 1 wherein carbon is used which is the residue of a preceding, calcium carbide dissociation. Q

7. A process as claimed-in claim-1 whe'rein theearbon has a specific surface'of atjleast 5000'- cmF/g.

' References Cited, by the Examiner UNITEDSTATESPATENTS' s/ss- Iaifeetali are. 7s 1o 4/ Menego zet al. -167 X OTHERREFERENCES Mikulinskii et'al.: Preparation of Calcium is' are De- Translation Series, US. Atomic Energy Comm.,, Ofiice of Technical Information, March'1961. i

, BENJAMIN HENKIN, l lri mc ry Examiner. v j j WINSTON A. DOUGLAS, Ex miner. 

1. A PROCESS FOR PRODUCING CALCIUM BY THERMAL DISSOCIATION OF COMMERCIAL CALCIUM CARBIDE WHICH COMPRISES HEATING A CHARGE CONSISTING OF A LOWER BED OF CALCIUM CARBIDE, WHICH CONTAINS CALCIUM OXIDE, AND AN UPPER LAYER OF CARBON PARTICLES HAVING A LARGE SURFACE AND A GRAIN SIZE OF ABOUT 0.5 TO 30 MM., IN A FIRST STEP AT A TEMPERATURE OF ABOUT 1500 TO 1800*C. AND AT A PRESSURE WHICH IS ABOVE THE EQUILIBRIUM PRESSURE OF THE REACTION 